This invention relates in general to the field of semiconductor wafer fabrication and, more particularly, to a dust cover having a film with an inorganic, anti-reflective coating and method of use.
Semiconductor manufacturing often involves a series of processes including deposition, photolithography and etching. During the photolithographic process, semiconductor manufacturers often use a photomask to copy an image of an electronic circuit on to a semiconductor wafer. Photomasks come in various sizes and shapes (e.g., a one times photomask or a reticle, which is a photomask that can be shot several times onto a single wafer with a photolithographic tool known as a stepper). Photomasks generally include a quartz blank with a patterned chrome layer deposited on one surface of the quartz blank. This patterned metal layer contains a microscopic image of an electronic circuit.
During the photolithographic process, this image is projected onto a wafer. If the image is projected several times onto a single wafer, the photomask is known as a reticle. As design rules have moved toward smaller and more dense integrated circuit (IC) devices, the quality of the projected image has become increasingly important. A poorly projected image may result in the manufacture of a non-functioning IC device. In many cases, a dust particle resting on the surface of a photomask or reticle, causes sufficient distortion of the projected image to render the IC device non-functioning.
As such, many, if not most, IC manufacturers rely on a dust cover film, which may on occasion be referred to as a pellicle, to keep dust from landing on the surface of the photomask. Typically, the film is attached to a frame, which may be attached to the photomask such that the film is held at a fixed distance from the surface of the photomask. During the photolithographic process, the image of any dust particle resting on the film will be out of focus on the wafers surface. As a result, the probability of a dust induced defect at the wafer surface is reduced.
Ideally, a dust cover film should be invisible to the radiant energy of the photolithographic tool. In order to produce clear, well-defined patterns, the film should preferably transmit nearly 100% of the radiant energy used. Most films, however, do not transmit 100% of the radiant energy. A small percentage of the radiant energy is reflected at the interfaces of the film""s surface and air. Generally, the amount of light reflected at the interface of two transparent layers (e.g., air and film) depends upon the refractive index, N, of the two layers. More precisely, the amount of reflection depends upon the difference between the N of the first layer and the N of the second layer. The greater the difference, the more reflection.
The index of refraction, N, for air is approximately 1.0. A dust cover film, on the other hand, may have an N of 1.5 or greater. This difference in index of refraction can result in an average reflection of approximately 8 percent of the light striking the surface at a normal incidence (90 degrees to the face of the film). Moreover, taking into account interference within the film, there may be peak reflections of approximately 16 percent. As a result, considerably less than 100 percent of the radiant energy used during the photolithographic process passes through the dust cover film.
Conventional techniques for combating this problem involve the addition of a film layer having a lower refractive index to a substrate film, which is usually made of nitrocellulose. The additional film layer is often spun onto the substrate film. As such, the substrate film needs to be relatively thick (approximately 1-2 xcexcm) to withstand the forces associated with the spinning process. Moreover, when thinner substrate films are used, damage to the substrate film during the spinning process becomes more likely.
Other conventional solutions include using a single layer of film with a relatively low refractive index. This conventional single layer approach does not allow for producing a dust cover film with a refractive index much below 1.4.
In accordance with teachings of the present disclosure, a dust cover film with an inorganic, anti-reflective coating and method of use are described. The described film and method of use provide significant advantages over prior technologies.
According to one aspect of the present disclosure a dust cover is described for use during photolithography. The cover includes a frame attached to a fluoropolymer film having an inorganic, anti-reflective coating. The frame may be manufactured from various materials including, for example, aluminum. The inorganic, anti-reflective coating preferably has a refractive index below 1.3. More preferably, the inorganic, anti-reflective coating has a refractive index between 1.13 and 1.2.
In one embodiment, the coated fluoropolymer film could be an amorphous fluoropolymer. The amorphous fluoropolymer may include, for example, copolymers of 30 to 99 mole percent perfluoro-2,2-dimethyl-1,3-dioxole (PDD) and complementary amounts of at least one comonomer. Preferably, the copolymers will have a glass transition temperature of at least 80xc2x0 Celsius.
The fluoropolymer film may also include an inorganic anti-reflective coating of calcium fluoride (CaF2). The coating may be applied in a number of ways, for example, physical vapor deposition. Other inorganic fluorides may also be used. For example, magnesium fluoride (MgF2) may be substituted for CaF2.
One method of manufacturing a coated film for use in a dust cover incorporating teachings of the present invention includes evaporating an inorganic coating material in a vacuum chamber and depositing it on a fluoropolymer film. The pressure in the chamber may be increased above normal to lower the refractive index of the resulting inorganic anti-reflective coating. A good approximate starting pressure may be 1xc3x9710xe2x88x924 torr. The factors affecting the appropriate pressure include, among others, source and type of fluoride, size and type of film, rate of deposition and desired refractive index.
The technical advantages of a dust cover incorporating teachings of the present invention include an increased transmission percentage as well as an increased durability (i.e., longer life cycle). These advantages may help improve efficiencies during a photolithographic process. In addition, the disclosed dust cover should have improved transmission across a broad spectrum of frequencies.
Another technical advantage of the present invention arises during manufacture of the dust cover itself. Conventionally, an anti-reflective coating is placed in solution and spun onto a substrate film. The dynamics of this systems as well as the solvents used when creating the solutions often damage the substrate film. An inorganic anti-reflective coating may be applied via physical vapor deposition, which should help reduce manufacturing problems.
Other technical advantages will be apparent to one of ordinary skill in the art in view of the specification, claims, and drawings.